Nutrition advice needed

Well, I've been all over the Web looking for some decent answers and the stuff I've come up with isn't news to me at all -- I've studied Tom Venuto, read two nutrition for the endurance athlete books and yet can not figure out just how much I should be eating.

I lost 70lbs while training for the three short races I did last year (it was all baby fat) and have another 20 to go in order to reach my optimal weight. Thing is, I've been on a plateau since November and nothing I seem to do will get me past it.

When I started to train in January for the long distance tris I'm doing this year, I kept getting sick and was constantly tired. Duh -- I was eating the same amount (my RMR = 1640cals) while upping my expenditure considerably. So, I'm currently eating between 2500 - 2900 cals a day because according to my Polar h/r monitor, I burn between 1200 - 1700 during workouts (11 hours a week of training).

Since I started eating more, I feel great! Haven't been sick a day (knock wood) despite the fact that dh and all four kids had a wierd throat thing with fever. BUT, my weight remains the same.

I think I may have put too much stress on my body by undereating and now it's adapting to the higher workout / caloric load. My question is, now that it's been a month of eating and working out like crazy, what can I do to shake things up a bit to lose that weight?

I need more wattage! My running speed is absolutely pathetic (9 km/hr on long distances) and although the bike is okay, I'd love to have that extra edge on the uphills that comes with less weight.

(BTW: No, I'm not an anorexic-type. I'm 6' tall and currently weigh 180lbs.)

Does ANYONE have any references on what to do? My first half-marathon is in April and I'd kill to do it at 11 km/hr!